The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Motor vehicles may include an engine system that produces drive torque that is transmitted through a transmission to a drivetrain to drive wheels of the vehicle. The engine system may be a hybrid engine system that includes an internal combustion engine and one or more electric machines. In various configurations, the engine and the electric machines may be independently coupled to the transmission. In such configurations, the engine, the electric machines, or a combination thereof, may be operated to supply torque to the transmission and thereby propel the vehicle.
Control systems have been developed to control the operation of the engine and the electric machines of hybrid engine systems. The control systems may control the operation to achieve improved fuel economy and reduced exhaust emissions. The control systems may control when the engine and the electric machines are operated. The control systems may also control the torque output by the engine and the electric machines.
To achieve improved fuel economy and emissions, the control systems may control operation using a closed-loop fueling process to more accurately control an air-to-fuel (A/F) ratio of the air and fuel combusted by the engine. Generally, engines are operated at or near a stoichiometric A/F ratio. The stoichiometric A/F ratio is defined as an ideal mass ratio of air to fuel for balanced combustion. The stoichiometric A/F ratio varies depending on the particular fuel used for combustion. The engine operates in a lean condition when the A/F ratio is greater than the stoichiometric ratio. The engine operates in a rich condition when the A/F ratio is less than the stoichiometric A/F ratio.
In the closed-loop fueling process, feedback from one or more sensors is used to more accurately control the A/F ratio. The feedback may be used to determine scaling factors that are used to adjust the amount of fuel delivered to the engine. The scaling factors are sometimes referred to as fuel trim values. Typically, one or more oxygen sensors located in an exhaust system of the engine are used as the primary feedback mechanism. Each of the oxygen sensors generates an output indicative of an oxygen content sensed in the exhaust. From the output, the control system can determine whether the engine is operating in a lean or rich condition. To function properly, the oxygen sensors must be operated at or above a temperature referred to as a sensitivity temperature. During periods when the oxygen sensors are not functioning properly, the control systems may control operation using an open-loop fueling process that does not obtain feedback from the sensors.